Apparatus Comprising an Avalanche Photodiode

ABSTRACT

Avalanche photodiodes are provided, wherein the APDs provide both high optical coupling efficiency and low dark count rate. The APDs are formed such that their cap layer has an active region of sufficient width to enable high optical coupling efficiency but the APD still exhibits a low dark count rate. These cap layers have a device area with an active region and an edge region, wherein the size of the active region is substantially matched to the mode-field diameter of an optical beam, and wherein the size of the edge region is made small so as to reduce the number of defects included. These APD designs maintain a substantially uniform gain and breakdown voltage, as necessary for practical use.

STATEMENT OF RELATED CASES

This case is a continuation of co-pending U.S. patent application Ser.No. 11/251,965 (Attorney Docket: 293-002US) filed 17 Oct. 2005, which isincorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to avalanche photodiodes in general and,more particularly, to avalanche photodiodes for use in single-photondetection applications.

BACKGROUND OF THE INVENTION

Avalanche photodiode (APD) structures that have separate absorption andmultiplication layers (SAM-APDs) can provide electrical output signalswith high fidelity (i.e., low noise). In a SAM-APD, the optical energyis absorbed and converted into electrical carriers in a layerspecifically designed for efficient absorption (hereinafter, referred toas the “absorption layer”). The resulting electrical signal is amplifiedin a different layer specifically designed for efficient electricalcarrier multiplication (hereinafter, referred to as the “cap layer”). Byseparating the absorption and multiplication functions into differentsemiconductor layers, each can be independently optimized for itsintended purpose.

A device region can be formed in the cap layer of a SAM-APD by diffusionof a dopant into the semiconductor layer to form a p-n junction. Theundoped portion of the cap layer that resides beneath the p-n junctionprovides a high-field region in which avalanche multiplication can occur(i.e., the avalanche multiplication region).

The principal driver for improved APD performance has been its use intelecommunications systems. For these applications, the APD iselectrically biased such that the electrical response is substantiallylinear with optical power. Recently, interest has arisen in the use ofAPDs for detection of single photons in such applications ascryptography. For single-photon-detection applications, the APD iselectrically biased at or beyond its “electrical breakdown voltage.” Thebreakdown voltage is the voltage at which the p-n junction issufficiently reverse-biased to conduct a large current arising from aself-sustaining avalanche process—even in the absence of continuousoptical power. An APD that is biased at or above breakdown, therefore,can give rise to an easily detectable pulse of electrical current inresponse to the absorption of even a single photon.

Two important parameters for an APD are the uniformity of the gain andbreakdown voltage across the device region. Gain and breakdown voltageare functions of the thickness of the undoped portion of the deviceregion. Dopant diffusion in a semiconductor is a substantially isotropicprocess (i.e., the dopant diffuses laterally and vertically, at nearlythe same rate). As a result, it is well understood that a diffused p-njunction will have a central portion (hereinafter, referred to as the“active region”) and an outer portion (hereinafter, referred to as the“edge region”).

The active region is characterized by a uniform, planar junction profilewhile the edge region has a non-uniform, curved junction profile. In theactive region, the uniform junction profile leads to uniform gain anduniform breakdown voltage. The curvature of the junction profile in theedge region, however, leads to a larger local electric field andtherefore higher gain and lower breakdown voltage than in the activeregion. This undesirable phenomenon is typically referred to as “edgebreakdown”. For practical SAM-APDs, the breakdown-voltage uniformityacross the entire device region should be within 10%, and preferablywithin 1%.

Another important performance metric for an APD, particularly in asingle-photon detection application, is Noise Equivalent Power (NEP).NEP is a function of the ratio of erroneous signals (referred to as thedark count rate) to optical detection efficiency. A photodiode with lowNEP will contribute few false counts while still detecting most or allof the received photons.

A low NEP can be achieved by 1) high detection efficiency and/or 2) lowdark count rate. Detection efficiency is a function of several factors:(i) the amount of the light signal which is directed into the detector(i.e., optical coupling efficiency); (ii) the probability that areceived photon is absorbed by the detector (i.e., quantum efficiency);and (iii) the probability that the absorbed photon will result in adetectable avalanche event (i.e., avalanche probability).

A high coupling efficiency can be achieved by making the device regionof an APD at least as large as the mode-field diameter of the opticalbeam. Many prior art photodiodes, in fact, have a device region that islarger than the mode-field diameter so as to both capture as much of thelight as possible and allow for some misalignment while still capturingthe entire beam.

Avalanche probability can be improved by increasing the bias voltage sothat it is well above the breakdown voltage. The larger this overbias,the greater the probability that a received photon will generate anavalanche event. Unfortunately, dark count rate also increases withoverbias; therefore in many cases increased overbias actually degradesNEP rather than improves it.

Device technologists in the communications field have long understoodthat device performance and manufacturing yield of semiconductor devicesare functions of material quality. In the past few decades, therefore,effort has been directed toward improving crystal growth techniques soas to reduce semiconductor material defect density. Improved crystalgrowth techniques can also reduce the presence of defects that serve asnucleation sites for dark current mechanisms. However, radicalimprovements in the overall materials growth technology area would berequired to affect any significant reduction of dark count rate.Moreover, it is unlikely that defects will ever be completely eliminatedthrough improved growth technique in a cost-effective manner.

It is desirable, therefore, to develop an avalanche photodiode withimproved NEP in a manner that is compatible with conventional crystalgrowth techniques and overcomes some of the costs and limitations of theprior art.

SUMMARY OF THE INVENTION

The present invention is an avalanche photodiode having separatemultiplication and absorption layers (SAM-APDs). In some embodiments,the avalanche photodiode provides high optical coupling efficiency andlow dark count rate. Some embodiments of the present invention areparticularly useful for single-photon detection applications.

An embodiment in accordance with the present invention provides an APDthat incorporates:

-   -   an active region of sufficient width for high coupling        efficiency with an optical beam; and    -   an edge region, wherein the width of the edge region is        substantially minimized, and wherein gain and breakdown voltage        in the edge region are within approximately 10% of the gain and        breakdown voltage, respectively, in the active region.

An aspect of the present invention is the inventors' recognition thatthe dark count rate is a function of the number of defects located inthe device region of an APD, and that the dark count rate can bedecreased by reducing the volume of the device region. In someembodiments, the total volume of the device region is reduced bydecreasing the width of the edge region. APDs of these embodiments stillmaintain a uniform gain profile or a uniform breakdown voltage profileacross the device region.

A further aspect of the present invention is the recognition that thecurvature of the junction profile in the edge region (and therefore thegain and breakdown voltage in the edge region) is affected by the ratioof certain parameters of the APD, as follows:

-   -   edge-region width to active region width;    -   edge-region width to the diffusion radius of the dopants in the        edge region;    -   edge-region width to cap layer thickness.        Some embodiments of APDs described herein fall within a desired        range of the foregoing ratios. Such APDs operate with low NEP        while maintaining a uniform gain profile or uniform breakdown        voltage across the device region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a SAM avalanche photodiode in communication with anoptical beam according to an embodiment of the present invention.

FIG. 2A depicts a cross-sectional view of an optical beam coupled to thecap layer of a SAM-APD according to an embodiment of the presentinvention.

FIG. 2B depicts a top view of an optical beam coupled to the cap layerof a SAM-APD according to an embodiment of the present invention.

FIG. 3A depicts a cross-sectional view of a cap layer of a SAM-APD,formed using a single dopant diffusion, according to the prior art.

FIG. 3B depicts a cross-sectional view of a cap layer of a SAM-APD,formed using a single dopant diffusion, according to an embodiment ofthe present invention.

FIG. 4 depicts the salient components of a single diffusion method forfabricating the cap layer of a SAM-APD in accordance with an embodimentof the present invention.

FIGS. 5A through 5C depict cross-sectional views of a cap layer duringproceeding stages of fabrication, as fabricated in accordance with theoperations depicted in FIG. 4.

FIG. 6A depicts a cross-sectional view of a cap layer of a SAM-APD,formed with a double diffusion, according to the prior art.

FIG. 6B depicts a cross-sectional view of a cap layer of a SAM-APD,formed with a double diffusion, according to an embodiment of thepresent invention.

FIG. 7 depicts the salient components of a double diffusion method forfabricating the cap layer of a SAM-APD in accordance with an embodimentof the present invention.

FIGS. 8A through 8D depict cross-sectional views of a cap layer duringproceeding stages of fabrication, as fabricated in accordance with theoperations depicted in FIG. 7.

DETAILED DESCRIPTION

The following terms are defined for use in this Specification, includingthe appended claims:

-   -   Device region is the portion of the cap layer defined by the        lateral extent of the doped portion of the cap layer. The device        region includes the full thickness of the cap layer, and        therefore includes the undoped portion of the cap layer that        resides under the doped portion. For example, the device region        includes both the active region and the edge region, as defined        below.    -   Active region means that portion of the cap layer wherein the        p-n junction has a substantially uniform doping profile and        junction depth (i.e., wherein the p-n junction is a planar        junction). The active region extends though the entire thickness        of the cap layer, and therefore also includes the avalanche        multiplication region under the planar junction.    -   Edge region means that portion of the cap layer wherein the p-n        junction does not have a substantially uniform doping profile        and junction depth (i.e., wherein the p-n junction is spherical        or cylindrical). The edge region extends through the entire        thickness of the cap layer. In APD designs that include guard        rings, the edge region includes those areas of the cap layer        that are doped to form the guard ring structures.

In the case of a single-photon detector, a low NEP is of particularimportance. A photodiode with low NEP contributes few false signalswhile still detecting many or most received photons. As described in theBackground section, a low NEP can be achieved by 1) high detectionefficiency, and/or 2) low dark count rate.

In some APD applications, particularly single-photon detection, thedesire for high detection efficiency and low dark count rate leads to aconflict in design methodology. In order to achieve high detectionefficiency, it is desirable that the device region of the APD is largeso as to couple all of the energy contained in the mode-field diameterof the optical beam being detected. On the other hand, the dark countrate is proportional to the size of the device region, and therefore itis desirable to make the device area small.

The present inventors recognized that for any given defect density, thenumber of defects is directly proportional to the volume of the deviceregion. As a result, the number of defects, and therefore the dark countrate, decreases with the volume of the device region. As described indetail below, APDs in accordance with the illustrative embodiment have areduced device-region volume. This is accomplished by reducing the widthof the edge region, while avoiding premature edge breakdown.

FIG. 1 depicts a SAM avalanche photodiode receiving an optical beamaccording to an embodiment of the present invention. Photodiode 100comprises substrate 102, absorption layer 104, charge control layer 106,and cap layer 108.

Absorption layer 104 absorbs the optical energy of optical beam 116, ascontained within mode-field diameter 118, and generates electricalcarriers. Absorption layer 104 is an intrinsic layer of indium galliumarsenide. It will be clear to those skilled in the art how to make anduse absorption layer 104.

Charge control layer 106 is a moderately n-doped layer of indiumphosphide. Charge control layer 106 enables maintenance of a lowelectric field in absorption layer 104, while supporting a high electricfield in cap layer 108. It will be clear to those skilled in the art howto make and use charge control layer 106.

Cap layer 108 is a lightly n-doped layer of indium phosphide. As usedherein, the terms “lightly n-doped” and “lightly p-doped” refer to theintentional doping of a semiconductor layer with an n-type or p-typedopant to a dopant concentration of less than three orders of magnitudegreater than the background concentration of the semiconductor layer.Within cap layer 108 is device region 110 which includes a heavily dopedp-type region. Device region 110 is formed by diffusing a p-type dopantinto cap layer 108 to form p-n junction 112. The lateral extent of p-njunction 112 defines the lateral extent of device region 110. Theundoped portion of device region 110 (i.e., the region beneath p-njunction 112) forms avalanche multiplication region 114, which is ahigh-field multiplication region of thickness W_(m)(x) in whichavalanche multiplication occurs.

Depending upon device design considerations, W_(m)(0), wherein x=0 isthe center of the active region, is a value in the range from 2% to 50%of the thickness of cap layer 108. In some embodiments, W_(m)(0) is avalue between 10-25% of the thickness of cap layer 108. In someembodiments, W_(m)(0) is approximately 20% of the thickness of cap layer108.

In some embodiments, cap layer 108 is a lightly p-doped layer of indiumphosphide and device region 110 is heavily doped with an n-type dopant.In some other embodiments, cap layer 108 is an intrinsic layer of III-Vsemiconductor material. It will be clear to those skilled in the art,after reading this specification, how to make and use embodiments of thepresent invention in which cap layer 108 is other than a lightly n-dopedlayer of indium phosphide.

FIGS. 2A and 2B depict a cross-sectional view and top view,respectively, of an optical beam coupled to the cap layer of a SAM-APDaccording to an embodiment of the present invention. Cap layer 108comprises device region 110, which is defined laterally by the extent ofp-n junction 112. Device region 110 comprises active region 202, havingactive-region width 206, and edge region 204, having edge-region width208.

Active region 202 is the central portion of device region 110 whereinp-n junction 112 is at a uniform depth (i.e., where p-n junction 112 isa plane junction). The value of W_(m)(x) is substantially equal toW_(m)(0) across active-region width 206. In some embodiments,active-region width 206 is substantially equal to mode-field diameter118 of optical beam 116. In some other embodiments, active-region width206 is made larger than mode-field diameter 118 in order to facilitateoptical coupling to optical beam 116.

Edge region 204 is the outer region of device region 110, wherein p-njunction 112 is non-planar. Edge region 204 is formed by the lateraldiffusion of the dopant used to form p-n junction 112, as will bedescribed below and with respect to FIGS. 5A through 5C. In edge region204, the value of W_(m)(x) and the profile of p-n junction 112 vary withx. In some embodiments, p-n junction 112 forms a quasi-cylindricaljunction in edge region 204. In some embodiments, p-n junction 112 formsa quasi-spherical junction in edge region 204.

In the embodiment of the present invention depicted in FIGS. 2A and 2B,active region 202 is a circular region surrounded by annular edge region204. In some alternative embodiments, active region 202 is non-circular.In embodiments wherein active region 202 is non-circular, edge region204 takes the form of a larger annulus of substantially the same outlinewith substantially uniform width.

FIG. 3A depicts a cross-sectional view of a cap layer of a SAM-APD,formed using a single dopant diffusion, according to the prior art. Caplayer 300 comprises semiconductor layer 302 and doped region 304.

Doped region 304 is a heavily p-doped region within n-dopedsemiconductor layer 302, and forms p-n junction 306. Doped region 304includes a planar junction region (i.e., an active region) havingactive-region width 308, and a cylindrical junction region (i.e., anedge region) having edge-region width 310. The junction depth of p-njunction 306 in the active region is approximately equal to diffusionradius r_(j). Active region width 308 is much larger than edge-regionwidth 310 and is also much larger than diffusion radius r_(j).

Avalanche gain, breakdown field, and breakdown voltage are allsubstantially uniform in the planar junction region, as denoted by theuniform space between equipotential electric field lines 312 in thisregion. The cylindrical junction nature of the edge region, however,gives rise to the well-known junction curvature effect. The junctioncurvature effect leads to a higher electric field intensity and lowerbreakdown voltage in the edge region (commonly referred to as “edgebreakdown”), as denoted by the crowding of equipotential field lines 312in regions 314. The effect of junction curvature on breakdown voltage isderived from Poisson's equation as:

$\begin{matrix}{V_{e} = {{\frac{V_{a}}{2}\left( {\eta^{2} + {2\; \eta^{6/7}}} \right){\ln \left( {1 + {2\; \eta^{{- 8}/7}}} \right)}} - \eta^{6/7}}} & \lbrack 1\rbrack\end{matrix}$

where:

-   -   V_(e) is the edge-region breakdown voltage;    -   V_(a) is the active-region breakdown voltage; and    -   η is a function of the radius of curvature of the junction in        the edge region, r_(j).

A number of approaches for limiting edge breakdown are known in theprior art. These include: 1) adding guard rings outside the junctionarea to control the doping density at the junction edges; 2) adding ashaped charge control layer underneath the cap layer to enhance theelectric field in the active region; and 3) forming a multi-tiereddoping profile to reduce the curvature (and therefore the induced localelectric field) of the junction profile at the edge of the deviceregion. Common to all these approaches is an enlarged device regionand/or more complicated device fabrication, which can lead to lowerdevice yield, higher device cost, and lower device reliability.

An aspect of the present invention is the recognition that the curvatureof the junction profile in the edge region is affected by severalfactors:

-   -   i. edge-region width with respect to active region width; or    -   ii. edge-region width with respect to the diffusion radius of        the dopants in the edge region; or    -   iii. edge-region width with respect to cap layer thickness; or    -   iv. any combination of i, ii, and iii.

As discussed earlier, the curvature of the junction profile affects theintensity of the electric field in the edge region, and, as a result,the edge-region breakdown voltage. The present invention, therefore,provides APD structures with specific relationships between:

-   -   a. the active-region width and the edge-region width; or    -   b. the diffusion radius of the dopant in the active region and        edge-region width; or    -   c. the active-region width and the thickness of the cap layer;        or    -   d. the edge-region width and the diffusion radii of the dopants        in the active and edge regions; or    -   e. the edge-region width and the thickness of the cap layer; or    -   f. any combination of a, b, c, d, and e.

In some applications, the mode-field diameter of the optical beam towhich the APD is to be coupled may be small. In many of theseapplications, the required active-region width could be small enoughthat the device region could be formed using a single dopant diffusionwhile still avoiding premature edge breakdown. The present invention,therefore, provides for APDs with device regions formed using a singlediffusion.

In other applications, the mode-field diameter of the optical beam towhich the APD is to be coupled is larger. In these cases, a largeractive region is required for efficient optical coupling. In the priorart, multiple diffusions (typically, two) have been used to form largeractive regions. The relative sizes of the multiple diffusion regionshave been determined, however, on the basis of avoidance of edgebreakdown and without regard to the size of the device region as awhole. Therefore, optical coupling efficiency has been pursued withoutconcern for dark count rate. In the present invention, the width of theedge region with respect to other design parameters is carefullyconsidered as one of the relationships outlined above. The relativeimportance of each of these relationships, however, can differ betweenthe single-diffusion case and multiple diffusion case. Therefore, thesingle-diffusion case and multiple diffusion case are addressedseparately below.

Formation of a Cap Layer Using a Single Diffusion

FIG. 3B depicts a cross-sectional view of a cap layer of a SAM-APD,formed using a single dopant diffusion, according to an embodiment ofthe present invention. Cap layer 108 comprises semiconductor layer 316and doped region 318.

Semiconductor layer 316 is a lightly n-doped layer of indium phosphidehaving a layer thickness of T_(c).

Doped region 318 is a heavily p-doped region within semiconductor layer316, which forms p-n junction 112. P-n junction 112 includes a planarjunction region (i.e., active region 202) having active-region width 206and a cylindrical junction region (i.e., edge region 204) havingedge-region width 208.

In some embodiments, semiconductor layer 316 is an intrinsicsemiconductor layer. In these embodiments, doped region 318 is dopedwith either an n-type dopant or a p-type dopant. In some embodiments,semiconductor layer 316 is a lightly p-doped semiconductor and dopedregion 318 is doped with an n-type dopant.

As will be described below and with respect to FIGS. 4 and 5A-C, dopedregion 318 is formed by diffusing a single p-type dopant intosemiconductor layer 316 using a single diffusion process. The p-typedopant diffuses into semiconductor layer 316 to a diffusion radius ofapproximately R_(de). In active region 202, the junction profile isplanar and the junction depth of doped region 318 is substantially equalto R_(de). In edge region 204, the junction profile is that of acylindrical junction having a radius of curvature of approximatelyR_(de).

It will be recognized by those skilled in the art that the lateraldiffusion rate of a dopant in a semiconductor sometimes differs from thevertical diffusion rate. As a result, the junction depth may differslightly from R_(de), even in the case of a single diffusion of a singledopant. For the purposes of this specification, however, a slightdifference in lateral and vertical diffusion rates is neglected, sinceit is not pertinent to the scope of the invention.

In some embodiments, multiple p-type dopants are diffused intosemiconductor layer 316 in a single process.

The local values of gain and breakdown voltage are functions of thelocalized electric field and, therefore, are affected by therelationship between active-region width 206 and edge-region width 208.In the case of an APD formed by a single diffusion, the crowding ofequipotential lines 320 becomes excessive as active region width 206 ismade larger than twice the diffusion radius, R_(de) (for example, caplayer 302 of FIG. 3A). The resultant reduction in breakdown voltage,therefore, also becomes excessive and the APD ceases to functionproperly. It is recognized as an aspect of the present invention, thatsufficient uniformity of breakdown voltage is achieved for an APD withactive-region width 206 in the range of approximately 0.5 R_(de) toapproximately 2 R_(de). In some embodiments, active region width issubstantially equal to R_(de). Alternatively, since edge-region width208 is approximately equal to R_(de), active-region width 206 can beexpressed as a function of edge-region width 208, wherein active-regionwidth 206 is in the range of approximately 0.5 to 2 times edge-regionwidth 208.

Diffusion radius R_(de) (and, therefore, the junction depth in activeregion 202) is chosen to provide a suitable avalanche gain in activeregion 202. In some embodiments, R_(de) is in the range of 0.5 T_(c) to0.98 T_(c). In some embodiments, R_(de) is in the range of 0.75 T_(c) to0.9 T_(c). In some embodiments, R_(de) is approximately 0.8 T_(c).Alternatively, since R_(de) can be expressed as a function of T_(c),active-region width 206 can be expressed as a function of T_(c), whereinactive-region width 206 is in the range of approximately 0.25 T_(c) toapproximately 2 T_(c).

FIG. 4 depicts the salient components of a single diffusion method forfabricating the cap layer of a SAM-APD in accordance with an embodimentof the present invention. In some embodiments, method 400 is used toform cap layer 108 shown in FIG. 3B.

The operations that compose method 400 are best described in the contextof a specific structure. To that end, the formation of cap layer 108, asdepicted in FIG. 3B, will be described in conjunction with FIGS. 5A-C.FIGS. 5A-C depict cross-sectional views of cap layer 108 at variousstages of fabrication.

At operation 401, a first mask-layer opening on a semiconductor layer isprovided, as depicted in FIG. 5A. Referring to FIG. 5A, mask-layeropening 502 is formed in mask layer 504. Mask layer 504 is a 200nm-thick film of silicon nitride deposited on semiconductor layer 316.Mask layer 504 is suitable for providing a diffusion barrier to p-typedopants. Mask-layer opening 502 is formed in mask layer 504 usingconventional photolithography and reactive ion etching. Other suitablemeans for forming mask-layer opening 502 include wet-etching, ionmilling, sputtering, and laser-assisted etching. It will be clear tothose skilled in the art how to make and use mask-layer opening 502 andmask layer 504.

At operation 402, p-type dopant 506 is diffused into semiconductor layer316 through mask-layer opening 502, as depicted in FIG. 5B. Suitablep-type dopants include, without limitation, zinc, cadmium, beryllium,and carbon. Referring now to FIG. 5B, dopant 506 diffuses bothvertically and laterally into semiconductor layer 316, and formsdiffusion front 508. The diffusion front in active region 202 forms as aplanar front that progresses downward vertically into semiconductorlayer 316. Lateral diffusion of dopant atoms at the edges of mask-layeropening 504, however, leads to the formation of edge region 204. Theprofile of diffusion front 508 in edge region is dictated by theclassical diffusion equation.

In some embodiments, p-type dopant 506 is implanted into semiconductorlayer 316 using ion-implantation. In some embodiments, ion-implantationis followed by a thermal treatment at an elevated temperature to drivethe implanted dopant further into semiconductor layer 316.

Referring now to the completed structure depicted in FIG. 5C, diffusionfront 508 and p-n junction 112 extend laterally past the edges ofmask-layer opening 502 by the distance equal to diffusion radius R_(de),thereby forming edge regions 204. In active region 202, the depth of p-njunction 112 is substantially equal to diffusion radius R_(de).

In some embodiments, semiconductor layer 316 is a p-type semiconductorand dopant 506 is an n-type dopant. Suitable n-type dopants include,without limitation, sulfur and silicon.

Formation of a Cap Layer Using Multiple Diffusions

FIG. 6A depicts a cross-sectional view of a cap layer of a SAM-APD,formed using a double diffusion, according to the prior art. Cap layer600 comprises semiconductor layer 602 and doped region 604.

Semiconductor layer 602 is a lightly n-doped layer of indium phosphide.

Doped region 604 is a heavily p-doped region within semiconductor layer602, and forms p-n junction 606. Doped region 604 includes a planarjunction region (i.e., an active region) having an active-region widthand a non-planar junction region (i.e., an edge region) having anedge-region width.

Doped region 604 is formed using two diffusion operations and thereforeincludes two diffusion fronts, first diffusion front 608 and seconddiffusion front 610. The junction profile of p-n junction 606 is afunction of both R_(da) and R_(de). R_(de) is the diffusion radius of afirst p-type dopant and R_(da) is the diffusion radius of a secondp-type dopant.

In the prior art, the edge-region width is often made quite wide toavoid premature edge breakdown near the active region. In addition, awider edge region creates a more smoothly varying junction profile inthe edge region and improves edge breakdown conditions.

FIG. 6B depicts a cross-sectional view of a cap layer of a SAM-APD,formed using a double diffusion, according to an embodiment of thepresent invention. Cap layer 612 comprises semiconductor layer 614 anddoped region 616.

Semiconductor layer 602 is a lightly n-doped layer of indium phosphidehaving thickness T_(c).

Doped region 616 is a heavily p-doped region within semiconductor layer614, and forms p-n junction 618. Doped region 616 includes a planarjunction region (i.e., active region 202) having active-region width 206and a non-planar junction region (i.e., edge region 204) havingedge-region width 208. Avalanche multiplication occurs in thehigh-electric field, undoped portion of active area 202.

In some embodiments, semiconductor layer 614 is an intrinsicsemiconductor layer. In these embodiments, doped region 616 is dopedwith either an n-type dopant or a p-type dopant. In some embodiments,semiconductor layer 614 is a lightly p-doped semiconductor and dopedregion 616 is doped with an n-type dopant.

As will be described below and with respect to FIGS. 7 and 8A-D, dopedregion 616 is formed by two diffusions of a p-type dopant intosemiconductor layer 614. First diffusion front 620 is a function of thediffusion radius, R_(de), of a first dopant and the diffusion radius,R_(da), of a second dopant. Second diffusion front 622 is at a distanceequal to the diffusion radius, R_(da), of a second dopant. The lateralextent of active region 202 is defined by the portion of seconddiffusion front which is at uniform depth (i.e., the planar junctionregion of p-n junction 618). The lateral extent of edge region 204 isdefined by the outer portion of p-n junction 618 that is not at uniformdepth. In order to provide a substantially minimum edge-region widthwhile also maintaining suitable edge breakdown characteristics,edge-region width 208 is in the range of approximately R_(de)+½ R_(da)to R_(de)+2 R_(da). In some embodiments, edge-region width 208 issubstantially equal to R_(de)+R_(da).

In some embodiments, the first dopant is the same as the second dopant.In some embodiments, R_(da) is equal to R_(de). It will be noted bythose skilled in the art that the lateral and vertical diffusion ratesof a dopant in a semiconductor are sometimes not equal. For the purposesof this specification, however, R_(da) and R_(de) will be treated asuniform in the lateral and vertical directions.

FIG. 7 depicts the salient components of a double diffusion method forfabricating the cap layer of a SAM-APD in accordance with an embodimentof the present invention. In some embodiments, method 700 is used toform cap layer 612 shown in FIG. 6B.

The operations that compose method 700 are best described in the contextof a specific structure. To that end, the formation of cap layer 612, asdepicted in FIG. 6B, will be described in conjunction with thecross-sectional views of cap layer 800 at various stages of fabricationdepicted in FIGS. 8A-D.

At operation 701, a first mask-layer opening on a semiconductor layer isprovided, as depicted in FIG. 8A. Referring to FIG. 8A, first mask-layeropening 802 is formed in first mask layer 804. The width of firstmask-layer opening 802 is first mask-layer opening width 806. First masklayer 804 is a 200 nm-thick film of silicon nitride deposited onsemiconductor layer 614, and is suitable for providing a diffusionbarrier to p-type dopants. First mask-layer opening 802 is formed infirst mask layer 804 using conventional photolithography and reactiveion etching. Other suitable means for forming first mask-layer opening802 include wet-etching, ion milling, sputtering, and laser-assistedetching. It will be clear to those skilled in the art, after readingthis specification, how to form and use first mask-layer opening 802.

At operation 702, p-type dopant 808 is diffused into semiconductor layer614 through mask-layer opening 802, thereby forming first diffusionfront 620, as depicted in FIG. 8B. During operation 702, dopant 808diffuses both laterally and vertically into semiconductor layer 614 asshown. Suitable p-type dopants include, without limitation, zinc,cadmium, beryllium, and carbon. In some embodiments, p-type dopant 808is implanted into semiconductor layer 614 using ion-implantation. Insome embodiments, ion-implantation is followed by a thermal treatment atan elevated temperature to drive dopant 808 further into semiconductorlayer 614.

At operation 703, a second mask-layer opening on semiconductor layer614, as depicted in FIG. 8C. Referring now to FIG. 8C, second mask-layeropening 810 is formed in second mask layer 812. Second mask-layer 812 isa 200 nm-thick film of silicon nitride, and is suitable for providing adiffusion barrier to p-type dopants. Second mask-layer opening 810 issmaller than first mask-layer opening 802 by an amount in the range ofapproximately R_(da) to approximately 4 R_(da). In some embodiments,second mask-layer opening 810 is smaller than first mask-layer opening802 by approximately 2 R_(da).

At operation 704, second p-type dopant 816 is diffused intosemiconductor layer 614 through second mask-layer opening 810, therebyforming second diffusion front 622. Due to the heat associated with thediffusion of second p-type dopant 816, first diffusion front 620progresses further into semiconductor layer 614 during operation 704. Insome embodiments, operation 704 comprises the ion implantation of seconddopant 816 into semiconductor layer 614. In some embodiments,ion-implantation is followed by a thermal treatment at an elevatedtemperature to drive dopant 816 into semiconductor layer 614 todiffusion radius R_(da) and to drive dopant 808 into semiconductor layer614 to diffusion radius R_(de).

Referring now to the completed structure depicted in FIG. 8D, diffusionfront 620 and p-n junction 618 extend laterally past the edges of secondmask-layer opening 810 by a distance that is a function of diffusionradii R_(da) and R_(de), and thereby form edge regions 204. Edge-regionwidth 208, therefore, is a function of R_(da) and R_(de), whereinedge-region width 208 is in the range of approximately R_(de)+½ R_(da)to approximately R_(de)+2 R_(da). In active region 202, the depth of p-njunction 618 is also a function of diffusion radii R_(da) and R_(de) andis in the range of approximately 0.5 T_(c) to approximately 0.98 T_(c).

In some embodiments, second mask-layer opening 810 and first mask-layeropening are both formed in first mask layer 804. In these embodiments,second mask-layer opening 810 is formed prior to first mask-layeropening 802.

In some embodiments, semiconductor layer 614 is a p-type semiconductorand first dopant 808 and second dopant 816 are n-type dopants. Suitablen-type dopants include, without limitation, sulfur and silicon.

It is to be understood that the above-described embodiments are merelyillustrative of the present invention and that many variations of theabove-described embodiments can be devised by those skilled in the artwithout departing from the scope of the invention. For example, in thisSpecification, numerous specific details are provided in order toprovide a thorough description and understanding of the illustrativeembodiments of the present invention. Those skilled in the art willrecognize, however, that the invention can be practiced without one ormore of those details, or with other methods, materials, components,etc.

Furthermore, in some instances, well-known structures, materials, oroperations are not shown or described in detail to avoid obscuringaspects of the illustrative embodiments. It is understood that thevarious embodiments shown in the Figures are illustrative, and are notnecessarily drawn to scale. Reference throughout the specification to“one embodiment” or “an embodiment” or “some embodiments” means that aparticular feature, structure, material, or characteristic described inconnection with the embodiment(s) is included in at least one embodimentof the present invention, but not necessarily all embodiments.Consequently, the appearances of the phrase “in one embodiment,” “in anembodiment,” or “in some embodiments” in various places throughout theSpecification are not necessarily all referring to the same embodiment.Furthermore, the particular features, structures, materials, orcharacteristics can be combined in any suitable manner in one or moreembodiments. It is therefore intended that such variations be includedwithin the scope of the following claims and their equivalents.

1-25. (canceled)
 26. A photodiode comprising a semiconductor layer forproviding avalanche multiplication of electrical carriers wherein thesemiconductor layer comprises: (1) an active region having asubstantially uniform breakdown voltage, V_(ba), and wherein the activeregion is doped with a first dopant to a first diffusion radius, R_(da),and wherein the active region is doped to a first junction depth that issubstantially uniform; and (2) an edge region having an edge-regionwidth, wherein the edge region is doped with a second dopant to a seconddiffusion radius, R_(de), and wherein the edge region is doped to asecond junction depth that has a second average junction depth, andwherein the edge-region width is within the range of approximatelyR_(de) to approximately R_(de)+2 R_(da), and further wherein the edgeregion has a breakdown voltage, V_(be), that is greater than or equal to0.9 V_(ba); wherein the active region and the edge region arecontiguous, and wherein the first junction depth is greater than thesecond average junction depth.
 27. The photodiode of claim 26 whereinthe active region and the edge region have substantially the same dopinglevel.
 28. The photodiode of claim 26 wherein the edge-region width issubstantially equal to R_(da)+R_(de).
 29. The photodiode of claim 26wherein V_(be) is greater than or equal to 0.99V_(ba).
 30. Thephotodiode of claim 26 wherein the active region has an active-regionwidth and the active-region width is less than or substantially equal to1.25 times the mode-field diameter of an optical beam.
 31. Thephotodiode of claim 26 wherein the active-region width is substantiallyequal to the mode-field diameter of the optical beam.
 32. The photodiodeof claim 26 wherein the first dopant and the second dopant are the samedopant.
 33. The photodiode of claim 26 wherein the semiconductor has athickness, T_(c), and wherein the first junction depth is within therange of approximately 0.5 T_(c) , to 0.98 T_(c).
 34. The photodiode ofclaim 33 wherein the first junction depth is within the range of 0.75T_(c) to 0.9 T_(c).
 35. The photodiode of claim 34 wherein the firstjunction depth is approximately 0.8 T_(c).
 36. A single-photon avalanchephotodiode comprising a semiconductor layer for providing avalanchemultiplication of electrical carriers wherein the semiconductor layercomprises: (1) an active region having a substantially uniform breakdownvoltage, V_(ba), wherein the active region is doped with a first dopantto a first diffusion radius, R_(da), and a second dopant to a seconddiffusion radius, R_(de); and (2) an edge region having a breakdownvoltage, V_(be), that is greater than or equal to 0.9V_(ba), wherein aportion of the edge region is doped with the second dopant to a seconddiffusion radius, R_(de), and wherein the edge region has an edge-regionwidth that is within the range of approximately R_(de) to approximatelyR_(de)+2 R_(da); wherein the active region and the edge region arecontiguous, and wherein the active region and the edge region havesubstantially the same doping level.
 37. The photodiode of claim 36wherein the semiconductor layer has a thickness, T_(c), and wherein theactive region is characterized by a first junction depth that issubstantially uniform and is within the range of approximately 0.5 T_(c)to approximately 0.98 T_(c), and wherein the edge region ischaracterized by a second junction depth that is non-uniform, andfurther wherein the second junction depth has an average junction depththat is less than the first junction depth.
 38. The photodiode of claim36 wherein the semiconductor layer has a thickness, T_(c), and whereinthe edge-region width is within the range of approximately 0.25 T_(c) toapproximately 2 T_(c).
 39. The photodiode of claim 38 wherein theedge-region width is in the range of substantially 0.5 T_(c) tosubstantially T_(c).
 40. The photodiode of claim 36 wherein thesemiconductor layer has a thickness, T_(c), and wherein the activeregion has an active-region width that is in the range of substantially0.25 T_(c) to substantially 2 T_(c).
 41. The photodiode of claim 40wherein the active region width is in the range of substantially 0.5T_(c) to substantially T_(c).
 42. The photodiode of claim 40 wherein theactive-region width is substantially equal to 0.8 T_(c).
 43. Asingle-photon avalanche photodiode comprising a semiconductor layer forproviding avalanche multiplication of electrical carriers wherein thesemiconductor layer comprises a device region that is formed byoperations comprising: doping the device region with a first dopant to afirst diffusion radius, R_(de), wherein the device region comprises anactive region that is surrounded by an edge region; and doping theactive region with a second dopant to a second diffusion radius, R_(da);wherein the active region has a substantially uniform breakdown voltage,V_(ba); wherein the edge region has a breakdown voltage, V_(be), that isgreater than or equal to 0.99V_(ba); wherein the edge region has anedge-region width that is within the range of approximately R_(de) toapproximately R_(de)+2 R_(da); and wherein the active region and theedge region are contiguous.
 44. The photodiode of claim 43 wherein theactive region and the edge region have substantially the same dopinglevel.
 45. The photodiode of claim 43 wherein the semiconductor layerhas a thickness, T_(c), and wherein the active region is characterizedby a first junction depth that is substantially uniform and is withinthe range of 0.5 T_(c) to approximately 0.98 T_(c), and wherein the edgeregion is characterized by a second junction depth that is non-uniformand has an average junction depth that is less than the first junctiondepth.